In the industrial field, the trend towards use of the Ethernet (registered trademark) has grown recently. At the manufacturing site, a communication network that connects Factory Automation (FA) devices such as a Programmable Logic Controller (PLC), an inverter, and a sensor has been constructed (hereinafter, the communication network is referred to as “FA network”). In general, in the FA network, the FA devices for which real-time operation is required are connected and controlled via the Ethernet. As the Ethernet-based FA network, the Control & Communication (CC)-Link IE (registered trademark) is commonly known.
In the CC-Link IE, a communication device (hereafter, “master”) that is a controller and a communication device (hereafter, “slave”) that is to be controlled are connected via the Ethernet to perform communication in accordance with a token passing method using multicast communication (for example, Patent Literature 1).
According to the communication system described in Patent Literature 1, a master regularly broadcasts a network-existence-confirmation frame to controlled-target slaves. On the basis of connection information of the slave included in an existence-confirmation-response frame from the slaves, the master detects an additional connection terminal and sets a token passing circuit.
After having determined the token passing circuit, the master notifies the slaves of a destination of the token. After having notified the slaves of the token passing circuit, the master multicasts data of its own and the token. A Media Access Control (MAC) address of a slave that is given transmission right for the next time is stored in the multicasted token. The slave, having received the token addressed to itself, recognizes that the slave has acquired the transmission right, transmits data as necessary, and thereafter multicasts the token to the next slave. In the CC-Link IE, the period of time from when the master transmits a token to when the master receives a token addressed to the master is controlled to fall within a preset time. By repeating the series of the processes, the master and the slaves perform cyclic communication that is periodic communication.
In an FA network configured by connecting various types of FA devices to a relay device such as a layer-2 switch, there is a possibility in that a loop path is formed by improperly connecting a communication path due to a human error or the like. In the FA network, a storm is caused by formation of the loop path. This hinders the FA-network operation. The storm refers to an infinite repetition of transfer and replication of a broadcast frame or a multicast frame on the loop path, by which the bandwidth of the FA network becomes insufficient. The FA network is required to be highly reliable and thus required to have a mechanism to autonomously detect and eliminate a loop path.
In the conventional CC-Link IE, a method to detect and eliminate a loop path within the FA network has been proposed. In this method, a relay device uses a network-existence-confirmation frame that is regularly transmitted from a master to determine that a loop path is detected at a port when the number of network-existence-confirmation frames received by the port within a set time is equal to or greater than a threshold, and then blocks this port (for example, Patent Literature 2).